Phase control thyristor with improved pattern of local emitter shorts dots

ABSTRACT

A phase control thyristor includes a main gate structure and a plurality of local emitter shorts dots arranged in a shorts pattern on a cathode side of the thyristor. The main gate structure includes longitudinal main gate beams extending from a center region of the cathode side towards a circumferential region. Neighboring main gate beams are arranged with a distance with respect to an associated intermediate middle line. The shorts pattern is more homogeneous in a region closer to a main gate beam than in a region closer to an associated middle line. Adaptions to match shorts patterns in neighboring segments of the cathode side surface are made in regions away from the main gate beams such that an electron hole plasma spreading from the main gate beam is not interfered by any inhomogeneity of the shorts dots pattern. The design rules enable an improvement of the thyristor operational characteristics.

RELATED APPLICATIONS

This application claims priority as a continuation application under 35U.S.C. §120 to PCT/EP2011/060329, which was filed as an InternationalApplication on Jun. 21, 2011 designating the U.S., and which claimspriority to European Application 10166682.4 filed in Europe on Jun. 21,2010. The entire contents of these applications are hereby incorporatedby reference in their entireties.

FIELD

The present disclosure relates to a phase control thyristor having amain gate structure as well as a plurality of local emitter shorts dotsarranged on a cathode side of the thyristor.

BACKGROUND INFORMATION

A thyristor, which is sometimes also referred to as silicon controlledrectifier (SCR), is a switching device which can be turned on whenforward biased by a turn-on voltage and when a positive gate current issupplied to a gate terminal. The thyristor is then said to be in aforward conducting state in which a current may flow from an anode to acathode. On the other hand, the thyristor can also be in a forwardblocking state meaning that a high positive voltage can be blocked. In areverse direction, the thyristor cannot be turned on. A thyristor designmay be reverse blocking, which means that it can block approximately thesame voltage in the reverse direction as in the forward off-state, orasymmetric, which means that it has virtually no blocking capability inthe reverse direction. Since phase control applications commonly requirereverse blocking capabilities, a phase control thyristor (PCT) isgenerally reverse blocking.

For high power applications, thyristors have been developed based onround semiconductor wafers having a diameter of, for example, 4 or 5inches. However, advanced thyristor applications utilize even largerthyristor designs based, for example, on 6 inch wafers. It has beenobserved that for such large thyristor designs, it may not be sufficientto simply scale-up previous smaller thyristor designs. With anincreasing thyristor diameter, further effects may gain influence on thethyristor operation. For example, a larger thyristor for higher nominalcurrent with equivalent forward blocking capacity or turn-oncharacteristics as well as cooling characteristics during thyristoroperation may not be achieved by proportionally scaling the thyristordimensions.

DE 1 954 665 discloses a rectifier having a star shaped main gatestructure. In between two stripes of the main gate structure, emittershorts are arranged in the form of a fir tree. There are no emittershorts arranged in a region close to the main gate structure. Such a firtree structure gives a coarse structure, which cannot be used for largerdevices because the distribution of the branches is too coarse toefficiently influence the blocking capacity and the turn-oncharacteristics in a way necessary for larger devices.

U.S. Pat. No. 4,903,105 discloses a triac including two thyristors. Onthe border between these thyristors, a maximum of two emitter shortstripes are arranged. The stripes may be formed as a chopped stripe,which have at least 30% of the stripe p doped, that is, the areasbetween the p doped regions may have a length of less than 70% of therepetition length of the p/n areas of the stripes. The arrangement ofthese emitter shorts between the thyristors is needed to separate thethyrisors from each other. This is a necessity for the operation of thetriac.

SUMMARY

An exemplary embodiment of the present disclosure provides a phasecontrol thyristor which includes a main gate structure on a cathode sideof the thyristor, and a plurality of local emitter shorts dots arrangedin a shorts pattern on the cathode side of the thyristor. The main gatestructure includes longitudinal main gate beams extending from a centerregion of a surface of the cathode side towards a circumferentialregion. Neighboring main gate beams are arranged with a distance withrespect to an associated intermediate middle line. The shorts pattern ismore homogeneous in a region closer to a main gate beam than in a regioncloser to an associated middle line.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the disclosure will be explained in more detailin the following text with reference to the attached drawings. Thedescribed embodiments are meant as examples only and shall not limit thepresent disclosure. The drawings are only shown schematically and arenot to scale. Generally, alike or similarly functioning parts are giventhe same reference symbols.

FIG. 1 shows a cross-sectional view of doping regions and contactarrangements of a phase controlled thyristor.

FIG. 2 shows a cross-sectional view of a phase controlled thyristor withan amplifying gate structure.

FIG. 3 shows a cross-sectional view of a portion of a phase controlledthyristor with cathode emitter shorts.

FIG. 4 shows a top view onto a shorts dots pattern of the phasecontrolled thyristor of FIG. 3.

FIG. 5 shows an n⁺-mask for defining phosphorous-doped regions (whiteareas) at a cathode side surface of a phase controlled thyristor with aconventional design.

FIG. 6 shows an enlarged view of portions of the mask shown in FIG. 5.

FIG. 7 shows an n⁺-mask for defining phosphorous-doped regions (whiteareas) at a cathode side surface of a phase controlled thyristor inaccordance with an exemplary embodiment of the present disclosure.

FIG. 8 shows an enlarged view of portions of the mask shown in FIG. 7according to an exemplary embodiment of the present disclosure.

DETAILED DESCRIPTION

Exemplary embodiments of the present disclosure provide a phase controlthyristor with a design enabling advantageous thyristor operationcharacteristics even with enlarged thyristor diameters.

According to an exemplary embodiment of the present disclosure, a phasecontrol thyristor includes a main gate structure on a cathode side ofthe thyristor, and a plurality of local emitter shorts dots arranged ina shorts pattern on the cathode side of the thyristor. Therein, the maingate structure includes longitudinal main gate beams extending from acenter region of the cathode side surface towards a circumferentialregion. Neighboring main gate beams are arranged with a distance withrespect to an associated intermediate middle line. Therein, the shortspattern is more homogeneous in a region closer to a main gate beam thanin a region closer to an associated middle line.

Exemplary embodiments of the present disclosure may be seen as beingbased on the following feature. As explained in further detail below,operation characteristics of a thyristor may depend on a multiplicity ofparameters. One of such parameters influencing, for example, theblocking capability and/or the turn-on characteristics of the thyristormay be the design or arrangement of a main gate structure as well as ofa spatial distribution of dot-shaped local emitter shorts at the cathodeside of the thyristor.

According to an exemplary embodiment of the present disclosure, the maingate structure may significantly influence the formation of anelectron-hole plasma during turn-on of the thyristor. Plasma formationmay start at the main gate structure and may spread laterally throughoutthe entire cathode side surface.

On the other hand, in accordance with an exemplary embodiment, aplurality of local emitter shorts may be provided in the form of dotsarranged throughout the remaining cathode side surface. The design ofthe shorts dot pattern may influence the forward blocking capacity bothstatically and dynamically as well as turn-on characteristics.

As the design of the main gate structure and the design of the shortsdots pattern may influence the thyristor operation characteristics indifferent ways, an improved design or layout of the cathode side of athyristor may have to take into account a trade-off between such variousinfluences. It has been found that advantageous thyristorcharacteristics may be realized by using a shorts pattern that is morehomogeneous in a region closer to a main gate beam than in a regioncloser to an associated middle line between neighboring main gate beams.In other words, the homogeneity of the dots pattern may be higher inregions close to a main gate beam than in regions further away from themain gate beam.

“Homogeneity” of the shorts pattern may mean that the dots forming theemitter shorts within one surface region of the thyristor cathode sidehave a more or less constant size and/or spacing. In other words,“homogeneous” may mean that the size of shorts dots and the distancebetween neighboring shorts dots is substantially the same throughout asurface region. This may also mean that a density of shorts dots issubstantially constant. “Less homogeneous” may then mean that at leastone of the size of dots or spacings between adjacent shorts dots maysignificantly vary throughout a surface region. The density of theemitter short dots may be expressed as a number of dots per surface areaor as a surface size of the dots per surface area. “Surface” shall bethat side of the dots toward the cathode metallization, for example, itis arranged in the cathode side plane.

In other words, it has been found to be advantageous that sizes ofshorts dots and spacings between adjacent shorts dots are substantiallyconstant within a region close to a main gate beam whereas such spacingsmay vary more in a region further away from the main gate beam, forexample, closer to an associated middle axis between neighboring maingate beams.

Similarly, sizes of the shorts dots may be substantially constant withina region closer to the main gate beam, whereas in a region closer to anassociated middle line, the sizes of the shorts dots may vary more.

This finding has been developed from the following understanding. Indesigning the cathode side of a thyristor including the main gatestructure and the shorts pattern, it has been observed that it maycommonly not be avoided that emitter shorts dots are arranged with acertain inhomogeneity in the shorts pattern. This may be due to the factthat the design of the main gate structure should be close to optimum inorder to optimize specific thyristor operation characteristics, while atthe same time such main gate structure design may also influence thedesign of an optimum shorts pattern. Thus, as the emitter shorts dotsmay not be provided with a homogeneous shorts pattern throughout theentire cathode side surface, it may be necessary to accept some patterninhomogeneity somewhere on the cathode side surface. It has beenobserved that such pattern inhomogeneity may be less detrimental tothyristor operation characteristics in case it is arranged as far awayfrom the main gate structure as possible. In other words, while patterninhomogeneities should be prevented in a region close to the main gatestructure, it may be accepted in farther regions such as in regionsclose to an associated middle line between neighboring main gate beams.

For example, shorts dots in the region closer to the main gate beam mayhave larger spacings than shorts dots in the region closer to theassociated middle line. For example, in the region closer to the maingate beam, the spacings and sizes of the shorts dots may be selectedsuch that the shorts dots pattern is close to optimum and the shortsdots density is close to an optimum low value. Such low density ofshorts dots, for example, at least one of small dots or large spacings,may improve, for example, plasma spreading initiated from the main gatestructure towards the remaining cathode side surface. On the other hand,far away from the main gate structure, the shorts pattern may vary assuch pattern inhomogeneity may be less harmful in such distant regions.As the shorts dots density should not fall below a specific optimumvalue, additional shorts dots may be arranged within such distant regionin order to “smoothen” a transition from one region close to a firstmain gate beam to an adjacent second region close to a secondneighboring main gate beam. Increased shorts dots density may then meanat least one of reduced spacings or increased sizes of the shorts dots.

The presence of shorts hinder unwanted triggering of the thyristor whenhigh dv/dt are applied. They also hinder plasma spread, when thethyristor is turned on by applying a gate pulse. Due to the fact thatthe shorts are placed homogenous, plasma spreads homogeneously. The onlyinhomogenities in the short pattern are along symmetry lines (associatedmiddle lines), between the amplifying main gate beams, where plasmaspread stops, because it reaches the symmetry line from oppositepositions. Thus, it is advantageous to place the inhomogeneities in theshort pattern close to the associated middle lines.

It may be advantageous that the neighboring main gate beams are arrangedsymmetrically with respect to the respective associated intermediatemiddle line. In other words, the middle line may be a line of symmetryfor the design of two neighboring main gate beams. For example, an evennumber of main gate beams may extend from the central region to thecircumference of the cathode side surface in the form of a star or asnowflake. The symmetry may simplify the design of the cathode sidesurface. The lines of symmetry may be a maximum distance away from theassociated main gate beams such that in the region of such symmetrylines, in homogeneities of the shorts pattern may be accepted. Ofcourse, an uneven number of main gate beams is also possible.

According to another feature of the present disclosure, the main gatebeams may be tapered. Therein, a beam width may reduce from the centerregion towards the circumferential region of the cathode side surface.Thus, the local width of the main gate beams may be adapted to a localcurrent density during regular thyristor operation. In other words, ametallization of the main gate beams may be designed such that thecurrent density is substantially kept constant throughout suchmetallization. Since turn-on current may be constant per turn-on linelength. This may lead to regularly tapered beams, for example, the beamwidth continuously reduces from the center region towards thecircumferential region. Thereby, space may be saved and a main thyristoremitter area may be optimized.

It may be noted that this feature of tapered main gate beams may also berealized independently of the above mentioned feature of thespecifically adapted shorts pattern. However, the proposed specificallyadapted shorts pattern according to exemplary embodiments of the presentdisclosure may allow for an increased freedom of the turn-on linegeometry such that an advantageous adaption of the geometry of the maingate beams may be easily obtainable.

According to another feature of the present disclosure, the phasecontrol thyristor includes a pilot thyristor on the cathode side, andthe pilot thyristor includes a pilot gate structure. This pilot gatestructure includes longitudinal pilot gate beams extending into a regionof the main gate beams. Since the emitter layer of the pilot may extendinto the broad part of the main gate beams, electron hole plasma fromthe pilot thyristor may extend into the main gate beams possiblypreventing overload in certain cases.

First, some basic principles of phase controlled thyristors (PCT) anddefinitions of terms and wordings subsequently used herein will be givenwith respect to FIGS. 1-4.

FIG. 1 schematically shows a cross-section of a simple thyristor 100.The thyristor includes four layers of semiconducting material havingalternating conduction types, for example, an npnp-layer-stackstructure. As used herein, an n-type conductivity may be referred to asa first type of conductivity, while a p-type conductivity may bereferred to as a second type of conductivity, which is different fromthe first conductivity type. In an order from a cathode side 102 to ananode side 104 of the thyristor 100, the thyristor first includes ann⁺-doped cathode emitter layer 106. Then, a p-doped base layer 108 andan n⁻-doped base layer 110 follow. Finally, at the anode side 104, ap-doped anode layer 112 is arranged. The n⁺-cathode emitter layer 106 iscontacted by a cathode metallization 114. The p-anode layer 112 iscontacted by an anode metallization 116. The p-base layer 106 iscontacted by a gate metallization 118.

When a positive voltage is applied between the anode metallization 116and the cathode metallization 114, the thyristor 100 may be switchedbetween a blocking state (off state) and a conducting state (on state).As long as no current is supplied to the gate metallization 118, thethyristor will remain in the blocking state. However, when the thyristoris triggered by supplying a current to the gate 118, electrons will beinjected from the cathode, flow to the anode where they will lead tohole injection, and an electron-hole plasma will form in the p-baselayer 108 and n-base layer 110 which may switch the thyristor to theconducting state. The conducting state may be maintained as long as theforward voltage is applied and will only be stopped when the voltageapplied between the anode metallization 116 and the cathodemetallization 114 is switched off or even reversed. Upon applying areversed, negative voltage between the anode and the cathode, thethyristor goes in its blocking state (off state) and may only beswitched to the conducting state (on state) by re-triggering by againapplying a gate current. However, in order to obtain its full blockingcapability, the reverse voltage has to be applied for a certain durationcalled quiescence time t_(q) such that the previously injectedelectron-hole plasma may disappear due to recombination processesthereby enabling the blocking capacity of the device.

To trigger a thyristor 100 as shown in FIG. 1, a substantial gatecurrent would be required. An easy improvement may include theintegration of an auxiliary thyristor, which are well-known to experts,between a main gate and an anode of a thyristor 100′ as depicted in FIG.2. The auxiliary thyristor may alternatively be called a pilot thyristoror an amplifying gate structure. Therein, the auxiliary gate 130contacts the p-base 108 in a region of the auxiliary thyristor 120. Theauxiliary thyristor 120 also includes a further n⁺-emitter layer 122.This further n⁺-emitter layer 122 is contacted by a further cathodemetallization 124 of the auxiliary thyristor. The further cathodemetallization 124 of the auxiliary thyristor is internally connected tothe main gate metallization 118 which contacts the underlying p-baselayer 108 in the region of the main thyristor 126. An n⁺-emitter layer106 included in the main thyristor region 126 is contacted by thecathode metallization 114 of the main thyristor. The further cathodemetallization 124 of the auxiliary thyristor 120 may not be accessiblefrom the outside of the thyristor 100′.

In accordance with an exemplary embodiment, the pilot thyristorstructure is integrated between central gate and main thyristor. At thecentral gate, the pilot thyristor has a further n+-emitter layer 122 andtowards the main thyristor a p+-emitter layer. These layers areconnected to each other via a metallization. The p+-emitter layer actsas a short on the border for the further n+-emitter layer 122. Thecurrent in the further n+-emitter layer 122 is converted via themetallization to a hole current, which again acts as an injectioncurrent for the main thyristor. The p+-doped emitter layer carries thehole current, which injects the opposite section of the main thyristor.A circumferential p+-emitter layer is sufficient for this purpose. Thecharge spreading is achieved via the metallization. Furthermore, theremay be shorts present in the further n+-emitter layer 122.

Whereas a high gate overdrive factor, for example, a ratio of utilizedgate current and minimum gate trigger current, may speed up triggeringof the thyristor 100′, a further improvement may substantially help thisprocess. As may be seen from FIG. 2, the triggered state of the mainthyristor 126, for example, the injected electron-hole plasma, starts ata boundary of the auxiliary thyristor 120 which might be a ring of about1 cm diameter at a center of the thyristor 100′. The plasma then has tospread out to the whole thyristor area which may take severalmilliseconds. Only after this will the thyristor exhibit its steady onstate forward voltage characteristic. To shorten the maximum distance tothe area elements of the thyristor device, a distributed amplifying gatestructure as shown in FIG. 5 and as described in more detail furtherbelow may be used. This means that a gate doping of the main thyristormay have a more complex structure such as shown according to anexemplary embodiment in FIG. 5 and may include, for example, a T-gatedesign commonly used for large area PCTs. Such T-gate design maysubstantially shorten the distance for plasma spread so that thethyristor may be fully turned on by about 1 ms after the gate triggeringpulse. Since plasma spread may be related to the time during which thereis already substantially forward current and still high blockingvoltage, this turn-on duration may have a strong influence on theturn-on energy loss.

A thyristor 100′ with a homogeneously doped n⁺-cathode emitter layer 106as shown in FIG. 2 may be very sensitive to transients with positivevoltage variations dv/dt. Without impeding the forward characteristicsignificantly, this disadvantage may be mitigated when small regions ofan n⁺-emitter layer are left out in the cathode emitter layer 106 andthe corresponding p-doped base layer 108 may reach the cathode sidesurface 102 metalized with the cathode metallization 114 as shown inFIG. 3. The p-doped regions with missing n⁺-doping on the cathode side102 may be referred to as cathode emitter shorts 128 as they mayshort-circuit the cathode junction. The emitter shorts 128 may form anohmic short-circuit across the p-base-emitter junction and may conduct asignificant portion of the current at low current densities, forexample, in all phases where forward blocking is required. Therefore, anundesired dv/dt triggering may be avoided in most practical cases.

As shown in the top view of FIG. 4, the emitter shorts 128 may beprovided in the form of small dots arranged in a regular pattern acrossthe whole cathode side surface 102. The shorts 128 not only influencethe axial triggering behavior, but a good shorts design may also yield ahigh lateral plasma spread velocity and may therefore result in a highpermissible current variation di/dt. It may be of high importance thatthere is no location with in which the shorts density gets below aminimum value because this location may form a weak spot duringre-application of forward blocking voltage after turn-off.

Exemplary diameters of the dots, for example, the largest extension ofthe dot on the surface, may be between 30 μm up to 500 μm, such asbetween 50 to 400 μm (e.g., between 100 to 400 μm). The surface area ofthe dots may be 2.5% to 20%.of the total surface area in the region ofthe cathode emitter layer alternating with the emitter short dots. Thatmeans that between 12 short emitter dots/cm² up to 30000 dots/cm² areplaced on the cathode side. In accordance with an exemplary embodiment,if the dots are small, more dots will be present than if the dots have alarge diameter.

In accordance with an exemplary embodiment, in the area closer to theneighboring main gate beam the dots will cover an area of 2.5% up to 8%of the total area, whereas in the area closer to the associatedintermediate middle line the density will be up to 20%, for example,between 8 and 20% or even between 10 to 20%.

In case the density is lower closer to the main gate beam, the number ofdots may be between 12 to 10000/cm², for example, at least 100/cm² up to3500/cm².

In case the density is higher closer to the associated middle line, thenumber of dots may vary between 40 to 30000, for example, at least200/cm².

Accordingly, it is evident that a design of the doping structure on thesurface of the cathode side 102 including the amplifying gate structureas explained above with respect to FIG. 2 as well as including theshorts pattern as explained above with respect to FIGS. 3 and 4 has tobe optimized in order to obtain satisfying operational characteristicsof the thyristor with respect to, for example, forward blockingcapability, triggering velocity, quiescence time and transientcharacteristics such as dv/dt stability.

In the following, exemplary embodiments of the present disclosure willbe described with reference to FIGS. 5-8. These drawings show diffusionmask patterns which may be used to define the n⁺-type emitter regions106 at the surface of the cathode side 102 of the thyristor 100. In thedrawings, the white regions indicate areas which may be doped withphosphorous in order to obtain the n⁺-type doped emitter layer 106, andthe dark regions prevent phosphorus deposition. FIGS. 5 and 6 show aknown mask design. FIGS. 7 and 8 show a mask design for a thyristoraccording to an exemplary embodiment of the present disclosure. Beforediscussing details of the disclosure by comparing structures of theknown design with the design according to an exemplary embodiment of thepresent disclosure, some considerations underlying such inventivedetails will be discussed.

The following disclosure mainly describes a number of qualitativeimprovements in designing the mask set for very large PCT designs. Itmainly addresses a lateral structure on the gated side of the thyristorand not any questions regarding starting silicon wafer design,diffusions, wafer edge contouring (e.g., beveling) or passivation. Thedesigns according to the present disclosure are especially useful forlarge thyristors with amplifying gate structures, independent of thevoltage class or device application. A further goal may be to obtain agood di/dt performance and low turn-on energy as well as a favorablerelation between on-state voltage and peak current, on the one side, andreverse recovery charge respectively circuit commutated recovery time,for example, quiecence time t_(q), at the other side.

In order to obtain full blocking capacity of a thyristor, the entireleakage current of the pnp-structure should be conducted on the cathodeside (e.g., hole current) at sufficiently low ohmic resistance throughthe p-contact areas of the p base layer 108 to the cathode metallization114 such that no electrons are injected. For this purpose, a highminimum density of emitter shorts 128 may be desired.

The turn-on process requires lateral spreading of the electron-holeplasma. Therefore, a suitable n⁺-emitter layer design has to be found inorder to inject electrons thereby generating a plasma front whichspreads laterally along the n⁺-emitter layer such that, finally, theentire surface of the connected main cathode emitter is switched to theconducting state. This process of plasma spreading may be disturbed bythe emitter shorts 128. Furthermore, the area of the emitter shorts 128may not contribute to the conducting process. Thus, in order to optimizethe turn-on process, a small density of emitter shorts 128 may beadvantageous.

The turn-on process may also depend on the length and the structuraldesign of the initially triggered contour. Therefore, particularly forvery large thyristor areas, this “turn-on line” should also be enlarged.However, for this purpose, a stronger triggering pulse may be required.Such pulse may not be provided by an external triggering device or “gateunit” directly. Therefore, large thyristors generally include aninternal triggering amplification. This may be achieved by at least onepilot thyristor. The gate unit triggers the pilot thyristor, and thepilot thyristor then triggers the main thyristor. Thus, on the cathodeside, three metal electrodes may be provided: (i) a central gate contactwhich may be round and which may be connected to the gate unit via athin wire; (ii) a cathode of the pilot thyristor in which the cathode isconnected to the gate fingers (extensions) of the extended gatestructure 306 and is electrically floating; and (iii) the main cathodewhich may be contacted by pressing a molybdenum disk thereon. Aseparation of the extended gate structure and the cathode sidemolybdenum disk may be obtained by providing the metallization of themain cathode with a larger thickness, thereby preventing contact of themolybdenum disk with the extended gate structure. However, for thecentral gate contact, an opening in the molybdenum disk may be required.

In the conducting state, the entire area below the main cathodemetallization 114 of the thyristor is flooded with charge carriers ofboth polarities, for example, electrons and holes, forming a plasma. Thephase control thyristor may be turned off passively by commutating ofthe current. As soon as the current changed its direction injectionstops on both sides and the plasma collapses approximately exponentiallyby recombination. The rate of recombination may be determined byinfluencing the carrier lifetime, for example, by irradiating thefinished wafer. In case the voltage between the main electrodes iscommutated again during this collapsing of the plasma, an increasedleakage current may flow which may lead to an automatic re-triggering.Only after characteristic time t_(q), also referred to as quiescencetime, a re-triggering may be prevented and the thyristor obtains itscomplete blocking capacity. This quiescence time may depend on thedensity of the previous plasma, the recombination rate and theefficiency of the bulk shorts distribution. Any local defect of the bulkshorts distribution may result in a local reduction of the threshold forre-triggering and may therefore result in an increased quiescence time.

However, also without any preceding current in forward direction, a fastincrease of the blocking voltage may result in an axial displacementcurrent due to a capacity of the space charge region of the thyristor.This may result in erroneous triggering and should be prevented by asufficient dv/dt stability. In contrast to the case of turning-off, thiscapacity current flows homogeneously from the entire area, for example,from the gate, the pilot and the triggering structure. Thus, the dv/dtstability may also limit the admissible triggering sensitivity of the“turn-on line” and requires a specific density of the shorts pattern atthe border of the main thyristor (and of the pilot thyristor at an innerside).

Accordingly, designing large area thyristors must balance differentinfluences:

(i) The forward voltage drop requires a low density shorts pattern andlong carrier lifetimes, whereas the quiescence time requires a specificminimum shorts pattern density and carrier lifetime limits.

(ii) The turn-on process requires a low density shorts pattern and longturn-on lines, while the forward voltage drop requires maximum area useand thus low area losses for the triggering structure.

(iii) The turn-on process requires a high triggering sensitivity of theturn-on line, while the dv/dt stability requires a restriction of thissensitivity.

(iv) A long turn-on line being not very sensitive for triggeringrequires a large pilot thyristor which then, however, means large arealosses being negative for the forward conducting state. Furthermore,cooling of the central region of the thyristor may be difficult as suchcooling may be realized by pressing the molybdenum disk onto the surfaceof the main thyristor in thermal contact.

FIGS. 5 and 6 show a known design of a mask 200 for defining then-emitter layer as it for a known thyristor having a diameter of 4inches. In known design approaches, a shorts pattern in a bulk region(subsequently referred to as bulk shorts pattern 202) has been designedand optimized first. Therein, the bulk shorts pattern was designedsubstantially homogeneously throughout the entire thyristor surface. Inthis bulk shorts pattern, the size of shorts dots 204 and the spacingsbetween neighboring shorts dots 204 was substantially constant. Afterdesigning such homogeneous shorts dots pattern, the extended gatestructure 206 had to be incorporated into the design. Thus, the bulkshorts pattern 202 had to be adapted in a region in proximity to thegate structure 206. As can be clearly seen in the enlarged view of FIG.6, additional shorts dots 208 have been incorporated along a border ofthe gate structure 206 in a region 210 close to the gate structure 206.

However, such adaption of the shorts pattern in a region close to thegate structure may result in a negative influence to the thyristoroperation characteristics. First, such region 210 is also responsiblefor the adaption of the quiescence time t_(q) and the dv/dt stability.Second, this region 210 has to be crossed by the plasma front duringturning-on of the thyristor. Taking into account that the efficiency ofthe shorts with respect to the quiescence time and the dv/dt stabilityis determined by the weakest point, it is evident that any adaption ofthe shorts pattern has to be performed conservatively, for example, itmay result in a locally over-dimensioned shorts density. However, duringturning-on, the plasma front has to overcome such extra barrierresulting from large shorts density. Furthermore, the complexity of theshorts pattern geometry may even increase in case of a more complexgeometry of the turn-on line.

Thus, it has been an idea underlying the present disclosure to decoupleany adaption of the shorts pattern from the geometry of the turn-on linein the neighborhood of the gate structure. In other words, thedistribution structure and the geometry of the turn-on line may berelieved from any restraints due to matching it with the bulk shorteningpattern. This may be achieved by a different and new orientation of thebulk shortening pattern and a new placement of any inevitable matchingzones into areas not being very sensitive to plasma spreading.

Accordingly, as shown in FIGS. 7 and 8, a new shorts pattern isprovided. In the design of the mask 300 according to the presentdisclosure, the shorts pattern is more homogeneous in regions 310 closerto a main gate structure 306 than in regions 312 closer to an associatedmiddle line 314, which is a symmetry line between neighboring main gatebeams 316 forming portions of the main gate structure 306. Thus, whilethe shorts pattern is substantially homogeneous in the neighborhood ofthe main gate beam, additional shorts dots 308 may be introduced in theregion 312 close to the middle line 314, thereby resulting in a densershorts dots pattern in this region. In this case, the density of theemitter short dots is smaller in a region closer to a main gate beam andlarger in a region closer to an associated middle line 314.

Alternatively, the shorts pattern is substantially homogeneous in theneighborhood of the main gate beam and less shorts dots 308 are presentin the region 312 close to the middle line 314, thereby resulting in alower shorts dots pattern in this region. With both alternatives, it isensured that the distribution of short dots is more uniform close to themain gate beam than far away from the main gate beams (e.g., on theassociated intermediate middle line).

Thus, the shorts pattern of the main thyristor area is not necessarily acontinuous pattern all around the distributing gate. However, the shortspattern along all parts of the turn-on line is absolutely regularleading to an even turn-on along the whole contour of the turn-onstructure. Adaptions of the shorts pattern are decoupled from theturn-on line and are moved into regions far away from the gate beamswhich regions do not have to be crossed by the plasma front. In otherwords, the regions where the different segments of the main thyristorareas meet and have to be matched may be either symmetry regions wheretwo plasma fronts meet or they are orthogonal to plasma spreading suchthat they, therefore, are of little influence for the plasma spreadingprocess and mainly irrelevant to di/dt capability.

With the new design rules, no or only minor inhomogeneities in theshorts pattern resulting from differences in shorts dots size orspacings between neighbouring shorts dots are realized in theneighbourhood of the main gate structure. Only in corners or at ends ofthe main gate structure, some minor adaption may be required.

With this simple design rule for adapting the shorts pattern, the gatestructure 306 may be designed with tapered main beams 316 having a beamwidth reducing from the center region towards the circumferential regionof the cathode side surface or the mask 300, respectively. Using suchtapered beams 316, the current density in the metallization of the beamsmay be kept constant. Since turn-on current may be constant per turn-online length, this may lead to regularly tapered beams that are savingspace and may optimize the main thyristor emitter area. It may be notedthat tapered main beams may also result in an advantageous distributionof the current density for a case of a conventional shorts patterns, forexample, where the shorts pattern has not been adapted as describedabove.

In accordance with an exemplary embodiment, the thyristor design of thepresent disclosure may also include a pilot thyristor on the cathodeside, wherein the pilot thyristor includes a pilot gate structure 318including longitudinal pilot gate beams extending into a region of themain gate beams 316. In other words, the pilot emitter extensions 320 ofthe pilot thyristor (which, in FIGS. 7 and 8 is represented by the wideareas of the central and beam structure) extends into the broad part ofthe main gate beams 316 allowing the plasma of the pilot thyristor toextend into the beams at high pilot thyristor currents preventingoverload in certain cases. Furthermore, this design may eliminate thebroadest parts of the masked cathode emitter layer, leading to a morehomogeneous point defect gettering of the whole thyristor area duringprocessing.

Summarized in a slightly different wording, the present disclosureprovides a lateral structure of large area thyristors with an extendeddistribution of the main thyristor turn-on line for good dl/dtperformance. First, the distribution structure and the geometry of theturn-on line may be relieved from any restraints due to matching it withthe area shorting pattern. This may be achieved by a different and neworientation of the area shorting and a new placement of the inevitablematching zones into parts irrelevant for plasma spreading. The newfreedom of turn-on line geometry may be used to match the width of thesecondary gate beams with the local turn-on current they have to carry,leading to conical instead of the usual equal-width design. This mayreduce the total area used for the beams (and lost in main thyristorarea) considerably for a given length of turn-on line, leading to a moreefficient use of silicon area. Then, the broadest parts of thedistribution beams adjacent to the central gate structure may be used toextend the pilot cathode emitter into the beams. This not only enablesthe pilot plasma to extend somewhat into the better cooled regionsremote from the center, it also equalizes the lateral surface dopingdistribution, thereby homogenizing the carrier lifetime gettering of thedevice.

The design according to the present disclosure may lead to the followingmain advantages of a large area thyristor made according to the presentdisclosure:

(i) With the exception of the corners, the turn-on line of the mainthyristor may be equally shorted all around, leading to a homogeneousturn-on along the whole turn-on line and precluding any preferential orhampered places at turn-on.

(ii) A minimum of main thyristor area may be lost for a given beamconfiguration and turn-on line length.

(iii) The pilot is somewhat protected against overload due to thepossible plasma spreading into the beams at peak pilot current.

(iv) During processing, the high surface doping may be more evenlydistributed across the wafer, leading to lower tension and morehomogeneous gettering action.

(v) The inevitable matching zones of the shorting may be moved to placesirrelevant for plasma spread and therefore can be clearly over-shorted,eliminating the risk of “high-t_(q)-spots”.

In accordance with an exemplary embodiment, the conductivity types ofall layers are switched, for example, the base layer 110 and cathodeemitter layer 106 are p type, and the base layer 108 and anode layer 112are n type.

It should be noted that the term “comprising” or “including” does notexclude other elements or steps and that the indefinite article “a” or“an” does not exclude the plural. Also, elements described inassociation with different embodiments may be combined.

It will be appreciated by those skilled in the art that the presentinvention can be embodied in other specific forms without departing fromthe spirit or essential characteristics thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restricted. The scope of the invention is indicated by theappended claims rather than the foregoing description and all changesthat come within the meaning and range and equivalence thereof areintended to be embraced therein.

List of Reference Signs

-   100 Phase controlled thyristor-   102 Cathode side-   104 Anode side-   106 n⁺ cathode emitter layer-   108 p base layer-   110 n base layer-   112 p anode layer-   114 Cathode metallization-   116 Anode metallization-   118 Gate metallization-   120 Auxiliary thyristor-   122 further n⁺ cathode emitter of auxiliary thyristor-   124 further Cathode metallization of auxiliary thyristor-   126 Main thyristor-   128 Shorts dots-   130 Gate of auxiliary thyristor-   200 Mask for n-region definition-   202 Bulk shorts pattern-   204 Shorts dots-   206 Distributed Gate structure-   208 additional shorts dots-   210 region close to gate structure-   300 Mask for n-region definition-   306 Gate structure-   308 additional shorts-   310 region close to main gate beam-   312 region close to middle line-   314 Middle line-   316 Main gate beam-   318 pilot gate structure-   320 pilot emitter extension into main distributed gate beams

What is claimed is:
 1. A phase control thyristor comprising: a main gatestructure on a cathode side of the thyristor; a plurality of localemitter shorts dots arranged in a shorts pattern on the cathode side ofthe thyristor, wherein: the main gate structure includes longitudinalmain gate beams extending from a center region of a surface of thecathode side towards a circumferential region; neighboring main gatebeams are arranged with a distance with respect to an associatedintermediate middle line; the shorts pattern is more homogeneous in aregion closer to a corresponding one of the main gate beams than in aregion closer to the associated middle line; and the density of theemitter shorts dots is smaller in the region closer to the correspondingone of the main gate beams and larger in the region closer to theassociated middle line.
 2. The phase control thyristor according toclaim 1, wherein the emitter short dots have a diameter between 30 to500 μm.
 3. The phase control thyristor according to claim 1, wherein theemitter short dots have a surface area of 2.5% up to 8% of the totalarea in the region closer to a main gate beam.
 4. The phase controlthyristor according to claim 1, wherein the emitter short dots have asurface area of 8% up to 20% of the total area in the region closer toan associated middle line.
 5. The phase control thyristor according toclaim 1, wherein a number of dots is between 12 to 10000/cm² in theregion closer to a main gate beam.
 6. The phase control thyristoraccording to claim 1, wherein a number of dots is between 40 to30000/cm² in the region closer to the associated middle line.
 7. Thephase control thyristor according to claim 1, wherein the shorts dots inthe region closer to a main gate beam have substantially the samespacings.
 8. The phase control thyristor according to claim 1, whereinthe shorts dots in the region closer to a main gate beam havesubstantially the same sizes.
 9. The phase control thyristor accordingto claim 1, wherein the shorts dots in the region closer to a main gatebeam have at least one of larger spacings and smaller sizes than theshorts dots in the region closer to the associated middle line.
 10. Thephase control thyristor according to claim 1, wherein neighboring maingate beams are arranged symmetrically with respect to the associatedintermediate middle line.
 11. The phase control thyristor according toclaim 1, wherein the main gate beams are tapered with a beam widthreducing from the center region towards the circumferential region. 12.The phase control thyristor according to claim 11, wherein the localwidth of the main gate beams is adapted to a local current densityduring regular thyristor operation.
 13. The phase control thyristoraccording to claim 11, wherein the beam width continuously reduces fromthe center region towards the circumferential region.
 14. The phasecontrol thyristor according to claim 1, comprising: a pilot thyristor onthe cathode side, the pilot thyristor including a pilot gate structurecomprising longitudinal pilot gate beams extending into a region of themain gate beams.
 15. The phase control thyristor according to claim 2,wherein the emitter short dots have a diameter between 50 and 400 μm.16. The phase control thyristor according to claim 2, wherein theemitter short dots have a diameter between 100 and 400 μm.
 17. The phasecontrol thyristor according to claim 4, wherein the emitter short dotshave a surface area of 10% up to 20% of the total area in the regioncloser to the associated middle line.
 18. The phase control thyristoraccording to claim 5, wherein the number of dots is between 100 to3500/cm² in the region closer to the main gate beam.
 19. The phasecontrol thyristor according to claim 6, wherein the number of dots is atleast 200/cm² to 30000/cm² in the region closer to the associated middleline.
 20. The phase control thyristor according to claim 2, wherein theemitter short dots have a surface area of 2.5% up to 8% of the totalarea in the region closer to a main gate beam.
 21. The phase controlthyristor according to claim 2, wherein the emitter short dots have asurface area of 8% up to 20% of the total area in the region closer toan associated middle line.
 22. The phase control thyristor according toclaim 3, wherein a number of dots is between 12 to 10000/cm² in theregion closer to a main gate beam.
 23. The phase control thyristoraccording to claim 4, wherein a number of dots is between 40 to30000/cm² in the region closer to the associated middle line.